1. Field of the Invention
The present invention relates to a thin-film transistor array substrate, a method of manufacturing the same, and a liquid crystal display device. Particularly, the present invention relates to a thin-film transistor array substrate to be used for a fringe-field switching mode liquid crystal display device, a method of manufacturing the same, and a liquid crystal display device.
2. Description of Related Art
A fringe-field switching (FFS) mode of a liquid crystal display device is a display technique that displays an image by applying a fringe electric field to liquid crystals filled between substrates placed opposite to each other. Because a pixel electrode and a counter electrode are formed by transparent conductive layers in the FFS mode liquid crystal display device, it is possible to obtain a higher aperture ratio and transmittance compared to an in-plane switching (IPS) mode.
In an FFS mode liquid crystal display device that has been used hitherto, at least six photolithography processes for forming (1) a counter electrode, (2) a gate electrode, (3) a semiconductor layer, (4) source and drain electrodes, (5) a contact hole and (6) a pixel electrode are necessary to manufacture a thin-film transistor (TFT) array substrate. This raises a concern that manufacturing costs are high compared to a twisted nematic (TN) mode in which a TFT array substrate can be manufactured generally by five photolithography processes.
In light of the above concern, Japanese Unexamined Patent Application Publication No. 2001-235763, for example, proposes a technique of reducing the number of photolithography processes with use of a half-tone mask.
However, process control of a photolithography process with use of the half-tone mask is more complicated than that of a normal photolithography process, and there is a problem with stable mass production. Further, the half-tone mask is more expensive than a normal mask, and there is also a problem with cost.
The present invention has been accomplished to solve the above concerns, and it is desirable to provide a thin-film transistor array substrate, a method of manufacturing the same, and a liquid crystal display device which enable reduction of the number of photolithography processes without use of a half-tone mask.